Monitoring house hold energy with CTs 2

[WaveEater]

I use KWH meters on my apartments to keep track of tenents electric usage. I have one unit that is not on a seperate sub-panel. Is it possible to connect a CT on each of the 8 circuits used for the one tenent space and wire all these Current Tranformers in series for each phase? This should give me a total watt hour usage when wired to my meter, correct?

thanks, Brad

 

[waross]

Use a window type CT.
Pass all the L1 (line one)wires through the CT.
Pass all the L2 wires through the CT in the opposite direction.
respectfully

 

[WaveEater]

That would make my life much easier. With my limited electrical knowledege I couldn't determine if passing all the L1 wires and L2 wires respectly through a CT would give me the Sum KWH or just the Average KWH of the circuits.

Thanks

 

[itsmoked]

Why not just pick up a w-hr meter at the local electrical supply or one from the web. They are thick as fleas now that all the big semiconductor companies have put out power meter chips.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi Keith.
As I understand the problem, he doesn't have a dedicated panel. I understand that he has about 8 individual circuits to monitor with no common point. Thus the use of a CT to combine the currents to use as an input to his KWHr meter.
WaveEater
The CT will not give you an average, it will give you the sum or difference. Thus any wires run through the wrong way will subtract.
I suggest connect the Kilo-Watt-Hour meter to L1 and L2 and connect the CT. Then run all the wires from L1 through the CT window and check the meter rotation. If the meter is running backwards, reverse the CT connections. Then run all the wires from L2 through the CT window the other direction.
Usually (but not always) the top row of breakers is L1 and the second row is L2.

The best way to check is to put one probe of the voltmeter on L2.
Check the breakers with the other probe.
Any breakers showing around 240 volts are L1.
Put one probe of the voltmeter on L1.
Check the breakers with the other probe.
Any breakers showing around 240 volts are L2.

Any breakers left over are either off, tripped or defective.
Either repair or replace, or determine the polarity from the position in the panel relative to the other known breakers.
If you have two phases (or three phases) of a three phase 208 V system this won't work. You may use two meters and two CTs, one set for each phase. Add the monthly consumption from both meters for the total consumption.
respectfully

 

[davidbeach]

Usually (but not always) the top row of breakers is L1 and the second row is L2.

As I read that I thought two rows, top and bottom, and it just couldn't work that way per the NEC. I think what was intended might be that first two breaker poles (#1 on the left and #2 on the right) are L1 then the next two (#3 & #4) are L2, and repeating L1, L2, until you get to the bottom.

Arbitrarily say that the upper left breaker is L1. Measure the voltage from that breaker to all other breakers (they have to be on). Breakers with zero volts to the upper left breaker are also L1, breakers with 240 volts to the upper left breaker are L2. The takes care of tandems and minis as well as normal breakers. Two pole breakers will have one L1 and one L2.

 

[waross]

Yes David, that was what was intended. Your method will work but for an engineer with limited familiarity with metering I chose a check method that covers all the bases.
As you know, many panels, when populated with 1/2" breakers, will have the top 4 breakers on line one.
Years ago the Square D XO line had breakers supported by both bus bars.
There was one breaker for each horizontal space and handle to the left was L1 and handle on the right was L2. Two pole breakers could face either way.
Before that, was the original Square D "Multi-Breakers" (tm)
I never installed a new one but I saw several replaced on upgrades and most electricians shops had one or two defective Multi-Breakers kicked into a corner.
There were several configurations but a typical Multi-Breaker was a large block with a vertical 2 pole 40 amp breaker (for an electric range) in the top center position. This would be flanked by 2 or 4, single pole 15 amp breakers. Facing the other way in the bottom center position would be a 2 pole 20 amp breaker (for electric hot water).
This may be flanked by single pole 15 amp breakers.
On each side was a large line lug for incoming power.
This was a single unservicable unit with multiple breakers.
If one breaker failed you either doubled up or replaced the entire "Multi-breaker".
These were the original Multi-Breakers and were identified as such on the nameplate.
Unfortunately the term "Multi-Breaker" was misapplied to describe conventional circuit breakers for decades even after the original Multi-Breakers were no longer even a memory.
(Thanks for the chance to ramble David.)
Respectfully

 

[ScottyUK]

Hi Brad,

One other thing: don't put CTs in series. Ever. Parallel the secondary windings to sum the currents. Pay attention to the polarity, usually marked by P1, P2 on the primary and S1, S2 on the secondary. If you invert one you will subtract that current rather than adding it to the others.


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Sometimes I only open my mouth to swap feet...

 

[itsmoked]

Might I suggest you go to this very well done site and use the Watthour meter wizard. Just step thru the questions.

You will likely be surprised by the low cost. I believe you can gang your coils too.

http://www.ezmeter.com/wizard.php

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[WaveEater]

Thanks guys, I have wired several of these CT Watthour meters. But, in the past I've always just had one pair of wires to monitor. Sounds like this one will involve putting a few more wires through each CT, No big problem. I greatly appreciate everyone’s advice

 

[stevenal]

A little detail on the method Waross uses: Each amp from L1 that returns on L2 gets counted twice. If potential is measured line to line, every watt is also counted twice. This is not a problem, it just changes your effective CT ratio. Adjust your multiplier accordingly.

 

[WaveEater]

Or I can just monitor the hot legs, not the neutrals. Correct?

 

[WaveEater]

sorry I mis read your post about monitoring L2 and L2. I do take the measured KWH and divide it by 2 to get the actual use.
This brings up another question. Since half of the 8 circuits are just off L1 ,(110 volt circuits) that just happen to be on the same leg, for that appartment. My .5 multiplier will be a little off.

 

[davidbeach]

stevenal, if L1 is one phase and L2 is the other phase, how do you figure that each amp gets counted twice? 120V circuits would go out on L1 and return on N or out on L2 and return on N. 240V circuits would, correctly, be measured twice.

 

[stevenal]

David,

L1 goes forward through the CT, while L2 goes through it in reverse. The amp sourced by L1 goes forward through the window, through the line to line connected load, and through the window again in the forward direction so the measured current is twice that of the load. If the monitored potential is line to line, the simple V*I*pf will result in twice the power measured. Instead, use V*I/2*pf. When the neutral is included, so that I1 is not equal to -I2, this arrangement has the effect of averaging the two currents. If voltage is balanced (this is the assumption we make to violate Blondel) then you will get the same result as with a two element (N-1) meter that independent monitors the two line currents and their associated line to neutral voltages.

 

[stevenal]

Here's the math:

For simplicity, pf=1

P=I1*V1n+I2*V2n is the Blondel solution

Voltage is balanced, so V1n=-V2n

P=I1*V1n-I2*V1n

P=V1n(I1-I2)

V12/2=V1n

P=V12(I1-I2)/2

The CT measures I1-I2. If the ratio is 1:1;

P=V12*Ict/2

Or back up a few steps and hook your meter's potentials L1 to neutral.

P=V1n*Ict

 

[davidbeach]

stevenal, got it. I had already mentally connected the potentials L1 (or L2) to neutral.

 

[bacon4life]

ScottyUK

In thread238-187587 the conclusion seemed to be that putting CT's in series was not common but that it would be reasonable in some circumstances. What is the key difference between these situations?

 

[davidbeach]

The only time you should connect CT secondaries in series is when the primaries are also connected in series. Any other configuration would be trying to have different currents flow through the two CT secondaries, but if they are in series that can't happen without breaking some electrical law someplace. Since the laws can't be broken, one of the CTs would be driven into saturation to allow the ratio between primary and secondary to be "broken".

 

[stevenal]

This situation is more like paralleling CTs. You could get the same result with a CT on L1 and another on L2 with the two secondaries paralleled so that L2 polarity is connected to L1 non-polarity.